In its annual State of the Nation's River report, the conservancy gave the waterway a D grade, down from D+ in its first report in 2007.
The conservancy says there are new concerns as well, including contaminants such as chemicals found in the river that have been linked to so-called intersex fish that have both male and female traits.
``The nation's river continues to face significant threats,'' said H. Hedrick Belin, president of the Potomac Conservancy.
More than 6 million people now live in the river's basin in Virginia, Maryland, West Virginia, and Pennsylvania, up 5 percent in the past five years. The river's vital signs have leveled off or declined as a result, the conservancy said, noting that in 2010 the river had the second largest decline in scores compiled by Chesapeake EcoCheck, a government university partnership, with four of six major health indicators declining.
The river, for example, is often unsafe for swimming after heavy rains because many sewage systems are tied to storm drains that overflow, a problem that only increases with the population, the conservancy said.
The conservancy said progress has been made in the past five years, but not enough to get ahead of the growing problems.
Many recommendations involve limiting runoff from developed areas. Forests also must be protected through conservation of existing areas and replanting, particularly along stream banks with a goal of a no-net loss of forests. The group is also calling for tighter toxic chemical controls.
Todd Lookingbill, an assistant professor of geography and the environment at the University of Richmond, said that local governments, ``need to step up in the coming months and lay out their plans for how they are going to reduce pollution in their communities.''
Lookingbill said it will pay ``tremendous dividends down the road in terms of clean and safe drinking water.''